The invention relates to an EUV-transparent interface structure for optically linking a first closed chamber and a second closed chamber whilst preventing a contaminating flow of a medium and/or particles from one chamber to the other.
The invention also relates to an EUV illuminating device comprising such an interface structure and to an EUV lithographic projection apparatus provided with such an EUV-transparent interface structure and/or illumination device. The invention further relates to a method of manufacturing devices wherein such an apparatus is used.
A lithographic apparatus is an essential tool in the manufacture of integrated circuits (ICs) by means of masking and implantation techniques. By means of such an apparatus, a number of masks having different mask patterns are successively imaged at the same position on a semiconductor substrate.
A substrate is understood to mean a plate of material, for example silicon, in which a complete multilevel device, such as an IC, is to be formed level by level by means of a number of successive sets of processing steps. Each of these sets comprises as main processing steps: applying a radiation-sensitive (resist) layer on the substrate, aligning the substrate with a mask, imaging the pattern of this mask in the resist layer, developing the resist layer, etching the substrate via the resist layer and further cleaning and other processing steps. The term substrate covers substrates at different stages in the manufacturing process, i.e. both a substrate having no level or only one level of already configured device features and a substrate having all but one level of already configured device features, and all intermediate substrates.
The minimum size of the device structures that can be imaged with the required quality by a lithographic projection apparatus depends on the resolving power, or resolution, of the projection system of this apparatus. This resolution is proportional to xcex/NA, wherein xcex is the wavelength of the projection beam used in the apparatus and NA is the numerical aperture of the projection system. To produce devices with a higher density, and hence higher operating speeds, smaller device structures have to be imaged so that the resolution should be increased. To this end, the numerical aperture could be increased and/or the wavelength decreased. In practice, an increase of the numerical aperture, which is currently fairly large, is not very well possible because this reduces the depth of focus of the projection system, which is proportional to xcex/NA2 and, moreover, it becomes too difficult to correct the projection system for the required image field. Therefore, the wavelength is reduced to decrease the minimum device feature that can still be imaged satisfactorily.
Conventional lithographic projection apparatuses employ ultraviolet (UV) radiation, which has a wavelength of 356 nm and is generated by mercury lamps, or deep UV (DUV) radiation, which has a wavelength of 248 nm or 193 nm and is generated by excimer lasers. More recently it has been proposed to use extreme UV (EUV) radiation in the projection apparatus. With such a radiation, also called soft-x ray radiation, which has a considerably smaller wavelength, considerably smaller device features can be imaged. EUV radiation is understood to mean radiation with a wavelength from a few to some tens of nm and preferably of the order of 13 nm.
Possible EUV radiation sources include, for example, laser-produced plasma sources and discharge plasma sources. A laser-produced plasma EUV source is described, for example, in the article: xe2x80x9cHigh-power source and illumination system for extreme ultraviolet lithographyxe2x80x9d in: Proceedings of the SPIE Conference on EUV, X-Ray and Neutron Optics and Sources, Denver, July 1999, Vol.3767, pages 136-142. A discharge plasma source is described in, for example, the article: xe2x80x9cHighly repetitive, extreme-ultraviolet radiation source based on gas-discharge plasmaxe2x80x9d in Applied Optics, Vol.38, No.25, Sep. 1, 1999, pages 5413-17.
EUV radiation sources, such as the discharge plasma source referred to above, require the use of a rather high partial pressure of a gas or vapor to emit EUV radiation. In a discharge plasma source, a discharge is created in between two electrodes, and ionized plasma generated is subsequently caused to collapse to yield very hot plasma that emits radiation in the EUV range. The very hot plasma is often created in xenon (Xe), since xenon plasma radiates in the EUV range around 13.5 nm. For an efficient EUV production, a typical pressure of 0.1 mbar is required near the electrodes of the radiation source. A drawback of having such a rather high xenon pressure is that xenon gas absorbs EUV radiation. For example, xenon gas with a pressure of 0.1 mbar transmits over 1 m only 0.2% EUV radiation having a wavelength of 13.5 nm. It is therefore required to confine the rather high xenon pressure to a limited region around the source. To this end, the source can be embedded in its own vacuum chamber that is separated by a chamber wall from the next vacuum chamber in which at least a part of the illumination optics, comprising a collector mirror, is arranged. Said chamber wall should have an EUV-radiation transparent opening to pass the EUV radiation from the source to the subsequent chamber, whilst the different vacuum levels in the source chamber and said next chamber should be maintained.
A problem with a discharge plasma source, but also with other plasma sources like a laser-produced plasma source, is the relatively large amount of debris, e.g. contaminating particles, produced by this source. For a discharge plasma source, the debris stems mainly from erosion of the electrodes, by interaction of the plasma with these electrodes, and from erosion of the walls of the source chamber. Such an erosion of the walls, which is due to the high temperature generated in the source chamber, also occurs in a laser-produced plasma source. Moreover, the plasma, like xenon plasma, may emit high-energetic ions. The contaminating particles and ions may escape through the opening in the wall of the source chamber and reach optical components, or reflectors of the illumination system. These components, the first one of which is a collector mirror, for example a grazing-incidence mirror, are composed of a number of thin layers of, for example, silicon and molybdenum. They are very vulnerable and their reflection coefficient is easily decreased to an unacceptable level when they are hurt by such particles. As a consequence, the intensity of the EUV illumination beam would become too small.
Another problem in a lithographic projection apparatus is that debris and byproducts, in general material, may be sputtered loose from the resist layer by the EUV beam. The evacuated intervening space between the substrate and the projection system allows the released material to migrate towards the projection system without undergoing substantial scattering or deflection. In the projection system, the material is deposited on one or more mirrors, thereby forming a spurious coating, which has a roughening effect on the mirror surfaces. As a result of this, the resolution and definition of the images formed by means of the projection rapidly degenerate. Moreover, the reflection coefficient of the mirrors decreases so that less EUV radiation can reach the resist layer.
It is an object of the present invention to provide means for solving the above-mentioned problems and to improve the performance of a lithographic projection apparatus.
According to a first aspect of the invention, it provides an EUV-transparent interface structure as defined in the opening paragraph, which is characterized by an EUV-transparent member and a gas guide for injecting a flow of EUV-transparent gas in the neighborhood of the member surface facing the contaminating flow and for flushing the EUV-transparent gas in a direction opposite to the direction of the contaminating flow.
The member blocks the flow of contaminating particles on their way to the vulnerable components. In order to be sufficiently transparent to the EUV radiation, this member should be relatively thin so that it may be destroyed by the high-energetic contaminating particles. To increase the lifetime of the member, an EUV-transparent gas is injected at the membrane surface that would be hit by the contaminating particles, and this gas flushes opposite to the direction of the contaminating flow so that it substantially keeps the contaminating particles away from the member. The member thus has to block only the very few particles that are not carried away by the flow of EUV-transparent gas. In this way, an effective and long-lasting block for contaminating particles is obtained.
The EUV-transparent interface structure is preferably further characterized in that the EUV-transparent gas is an inert gas.
Such an inert gas, like helium or argon absorbs only little of the EUV radiation. Since only few electrons of the helium gas can be excited by the EUV radiation, this gas is very transparent to EUV radiation. Argon molecules are larger than those of helium gas. This means that argon gas, on the one hand, better captures the contaminating particles and, on the other hand, has a somewhat lower transparency than helium gas.
According to a further preference, the EUV-transparent interface structure is characterized in that the gas guide is constructed to inject the flow of EUV-transparent gas in at least two diametrically opposite directions.
An optimum gas flow along the membrane surface and towards the source of contamination is thereby obtained.
The EUV-transparent structure is preferably further characterized in that it comprises a hollow tube in the form of a cone, the narrowest opening of which faces the contaminating flow.
Such a cone-shaped tube has the minimal volume which is necessary to encapsulate the EUV radiation. This is advantageous, because it minimizes the flow of EUV-transparent gas required for producing an effective flush, leading to gas savings and a lower absorption.
A first embodiment of the EUV-transparent interface structure is characterized in that the member is a membrane.
This embodiment is preferably further characterized in that the membrane is made of silicon.
A thin membrane of silicon shows the required EUV-transparency. The membrane may also be made of zirconium or boron.
A second embodiment of the EUV-transparent interface structure is characterized in that the member is a channel structure comprising adjacent narrow channels separated by walls, which are substantially parallel to a propagation direction of EUV radiation to be transmitted by the channel structure.
This embodiment is especially suitable to maintain different vacuum levels in adjacent vacuum chambers.
The second embodiment is preferably further characterized in that the width of the channels increases or decreases in said propagation direction in accordance with a diverging or converging shape of the beam of EUV radiation.
The channel structure can then transmit a maximum amount of EUV source radiation.
The second embodiment may be further characterized in that the channel structure comprises a honeycomb structure.
The second embodiment is preferably characterized in that a cross-sectional dimension of said channels in a radial direction perpendicular to said propagation direction is larger than a cross-sectional dimension of said channels in a tangential direction around said propagation direction.
The invention also relates to an EUV illumination device, which comprises an EUV radiation source arranged in a first chamber and an optical system for receiving EUV radiation from the first chamber and transforming this radiation into an EUV radiation beam, the optical system being arranged in at least a second chamber. This illumination system is characterized in that an EUV-transparent interface structure as described hereinbefore is arranged between the first and the second chamber and in that the flow of EUV-transparent gas is flushed towards the source.
The components of the optical system, especially a first, collector, mirror are well protected against contaminating particles so that their reflection coefficient remains at an acceptable level during the lifetime of the illumination system.
The invention also relates to an EUV lithographic projection apparatus, which comprises an illumination device supplying an EUV beam for illuminating a mask, a mask holder for holding the mask, a substrate holder for holding a substrate and a projection system arranged between the mask holder and the substrate holder for imaging a mask pattern on the substrate by means of the EUV beam. This apparatus is characterized in that the projection system comprises at least one EUV-transparent interface structure with a membrane as described herein before.
The components of the projection system may be arranged in more than one chamber, while each chamber may be at a different vacuum level. Any one of the chambers may be provided with the EUV-transparent interface structure of the present invention, although the amount of debris in intermediate chamber(s) is considerably smaller than in the last chamber facing the substrate.
An optimum use of the invention is made in an apparatus, wherein the substrate is arranged in a substrate chamber and which is characterized in that an EUV-transparent interface structure is arranged between the last chamber of the projection system and the substrate chamber, and in that the flow of EUV-transparent gas is flushed towards the substrate.
The components of the projection system, especially those arranged in said last chamber, but also those arranged in other chambers are now protected against particles freed from the resist layer by the EUV radiation.
This embodiment is preferably further characterized in that its illumination device is a device as described herein before.
The whole projection apparatus is then well-protected against contaminating particles from both the radiation source and the resist layer on the substrate.
Finally, the invention also relates to a method of manufacturing a device comprising device features in at least one level of a substrate, which method comprises at least one set of the following successive steps:
forming a patterned coating on the substrate, the pattern of which corresponds to the device features in the level to be formed, and
removing material from or adding material to areas of said device level being formed, which areas are delineated by the pattern of the patterned coating. This method is characterized in that the patterned coating is made by means of an EUV lithographic projection apparatus as described herein before.
By use of the invention, the lithographic projection process becomes a reliable process and the manufactured devices have a constant, good quality.
These and other aspects of the invention are apparent from and will be elucidated by way of non-limitative example with reference to the embodiments described hereinafter.
In the drawings:
FIG. 1 schematically shows a prior art embodiment of a lithographic projection apparatus for repetitively imaging a mask pattern on a substrate, in which the invention can be used;
FIG. 2 schematically shows the various vacuum chambers of a lithographic projection apparatus;
FIG. 3 schematically shows an embodiment of an EUV-transparent interface structure used in this apparatus;
FIG. 4 shows a preferred embodiment of this interface structure;
FIG. 5 shows the application of the interface structure in an illumination device;
FIG. 6 shows the application of the interface structure between the projection system and the substrate;
FIG. 7 shows a channel structure for use in the EUV-transparent interface structure;
FIG. 8 shows a part of a radiation source/illumination unit provided with this interface structure, and
FIG. 9 shows a part of the channel structure having a honeycomb shape.